The present invention relates generally to methods for the conversion of fullerenes to diamond or diamond films. More particularly, the invention is concerned with the manufacture of nonhydrogenic fullerenes as precursors for the synthesis of diamond or diamond-like films. Because of their thermodynamic instability with respect to diamond, the conversion of fullerenes to diamond has significant advantages over presently used methods for producing synthetic diamonds.
The prior art methods of manufacturing diamond can be divided into two main approaches. In the first or high-pressure method, graphite powder is heated to about 1500.degree. C. under 60 kbar of pressure in the presence of an iron catalyst. Under this extreme, but equilibrium, condition of pressure and temperature, graphite can be converted to diamond. About 75 tons of diamond are produced industrially each year in this way. The second or low pressure method of producing diamond artificially involves producing a mixture usually of a few percent of methane in hydrogen gas. A hot filament or a microwave discharge is used to dissociate the methane molecule to form the methyl radical, CH.sub.3, and the hydrogen molecule is dissociated to form hydrogen atoms. Diamond or diamond-like films can be grown this way epitaxially on diamond nuclei. Such films, however, always contain small contaminating amounts (0.1-1%) of hydrogen which are undesirable for many applications.
The usefulness and novelty of fullerene precursors for diamond synthesis stem from several of their properties: Fullerene precursors are thermodynamically unstable with respect to diamond and, therefore, stable only in a kinetic sense. In addition, since the fullerenes are molecular entities, they are volatile with C.sub.60 having a vapor pressure of 10.sup.-4 Torr at 500.degree. C. Fullerenes are also allotropes of carbon; that is, they contain no hydrogen; and therefore, diamonds produced from fullerene precursors are hydrogen-defect free. Another useful characteristic of fullerene is the chemical bond in C.sub.60 is intermediate between graphite (sp.sup.2) and diamond (sp.sup.3). Furthermore, fragmentation involving carbon-carbon bond breakage occurs via the elimination of C.sub.2 groups. Recent scanning tunneling microscope studies have shown C.sub.2 groups to be intimately involved in the growth of epitaxial diamond films, particularly of the "dimer rows" of the 2.times.1 reconstructed &lt;100&gt; surface. It has also been determined both the positive C.sub.60.sup.+ and negative C.sub.60.sup.- ions are stable entities that can be accelerated to kilovolt energies under an applied electrostatic potential. The so-called LUMO (lowest unoccupied molecular orbital) of the fullerenes also is an antibonding three-fold degenerate orbital that can accept up to six electrons from electron donors such as the alkali metals. The resultant repulsion between the delocalized electrons weakens the carbon-carbon bonds of the C.sub.60 cage and provides a mechanism for the fullerene to undergo diamond transformation.